The present invention relates to a pressure spring, preferably a helical valve spring, for assembly in engines, pumps or the like, in which it is compressed to a first extent in a first operating state, e.g., when the valve is closed and to a second extent which exceeds the first in a second operating state, i.e., when the valve is open.
If, for example, this type of valve spring is installed in an internal combustion engine of the type used in automobiles, the value will be closed in the first operating state during which a certain amount of compression takes place and it will be open in the second operating state in which the valve spring is compressed to a still greater extend. In the course thereof the following problem arises: after completion of a first stroke between the first and the second operating state or condition, which the valve spring carries out under the force of the sinusoidal motion of the camshaft, i.e., after closing of the valve, the spring exhibits in its first operating condition oscillations of the inner tangential stress .tau..sub.k, which occur with the inherent natural frequency of the spring. These oscillations only die out after some time has lapsed (fade-out). This fade-out should have been ended before the next stroke begins, since only then no increase of the inner tangential stress can be caused at this time by the oscillation and only then a satisfactory operation of the valve springs can be ensured. However, with known valve springs this has not been possible in case of high rotational speeds of the engines. At high rotational speeds, the amplitude of these oscillations at the fade-out will become rather high; this results in the fact that the fade-out has not yet come to an end, when the next stroke begins. This again results in an increase in the upper tangential stress .tau..sub.ko by the amount of the amplitude of the oscillation at the beginning of the subsequent stroke. This corresponds to the residual energy which still remains in the spring at the beginning of the next stroke.
This increase in the inner tangential shearing stress is essentially undesirable as it represents increased loading of the spring and thus should be avoided. The vibrations per se are also generally undesirable as they can cause incorrect closing of the valve.
The object of the present invention is to design a pressure spring of the type described initially which can be incorporated in automobile engines as a valve spring in such a way that is has improved speed characteristics over the known pressure springs. Characteristic behavior at high rotational speeds is considered to be "improved," if, at the closing of the valve, an increase in the maximum inner tangential stress .tau..sub.ko, as caused by oscillations of the inner tangential stress, is essentially avoided. This problem is solved by providing the spring with windings, which differ from adjacent windings in terms of diameter and slope in such a manner that a transitional zone or more than just one transitional zone respectively are formed between these windings and the adjacent ones, and that the transitional zones contact or rest against the adjacent windings over a predetermined area. This contact should at least be present in the second condition of operation. Thereby through the frictional contact of the transitional zone and the adjacent winding, a damping of the oscillations of the inner tangential stress occurs, which leads to a considerably quicker fade-out of these oscillations than with known springs, i.e., without damping by frictional contact. As a result, the aforementioned disadvantages of the known springs are avoided.
Helical springs with inwardly-turned end coils are already known (German Pat. No. 1,169,209). However, in the case of the latter, emphasis is placed on ensuring that the end coils of the spring do not contact the adjacent coils during operation. The present invention, on the other hand, is based on the concept that friction is possible without disadvantageous consequences and that it can be used to dampen the vibrations of the inner tangential stress. Insofar as inwardly-wound end coils are otherwise known (German Offenlegungsschrift No. 2,258,572), they are used to allow for reduced height dimensions in a state, in which the spring is blocked, i.e., when all the windings rest against one another. The latter German disclosure makes no reference to avoiding vibrations or oscillations, which occur with the inherent natural frequency.
Among the various advantageous features of the invention are the following:
Friction damping is obtained by the fact that the transition zone or transition zones respectively contact or rest against the adjacent windings. This results in lower oscillation amplitudes of the inner tangential stress and a more rapid decline of the same, i.e., a quicker fade-out. The oscillatory energy, which still is present at the beginning of the next stroke, is correspondingly less and accordingly the increase in the maximum tangential stress .tau..sub.ko, which occurs at extremely high rotational speeds, is also considerably reduced as compared with known valve springs. As a result, the characteristic behavior at high rotational speeds is substantially improved.
It is one advantageous feature of the invention that the end windings are disposed to lie in a plane, which extends perpendicular to the axis of the spring. Thereby it is possible to avoid to grind a surface at the end of the end windings to obtain an abutting surface, against which any support of the spring may rest.
The aforementioned advantages also make it possible to employ a symmetrical configuration without incurring the disadvantages which might be expected. As a result of the symmetrical configuration it is not necessary to specify a particular assembly direction while an advantageous distribution of tension within the spring is obtained.
The relatively high inherent frequency selected according to an advantageous feature of the invention is higher than the inherent frequency of known valve springs. In combination with the marked damping of the vibration amplitude it is presumably responsible for the fact that the energy released when the valve is closed is reduced more rapidly than in the case of the known valves.
The relatively large looping angle according to an advantageous feature of the invention provides for a good support and good seating of the valve spring in other machine elements, in which the spring is received (rotocap and valve plate).
Other objects, features and advantages of the present invention will be made apparent in the course of the following description thereof which is provided with reference to the accompanying drawings.